The present disclosure is related to solid state light emitting devices, and more specifically to laser devices emitting in the visible (e.g., 400 nm-550 nm) portion of the spectrum.
Solid state light emitting devices are known today, and are used in many different applications. For many such applications, the desired output wavelengths may be obtained from the devices themselves by current injection. For other such applications, the output of the light emitting device must be modified to obtain the desired wavelength. For example, in order to obtain output light in the deep-UV (e.g., 200 nm-300 nm), no single-chip, electrically driven device has been shown that provides adequate desired wavelength, power, beam quality, and lifespan. Deep-UV semiconductor-based laser devices that have been investigated so far are generally gallium-nitride-based ridge-waveguide laser diodes that inherently have low beam quality and exhibit low output power. Due to challenging materials issues in nearly every aspect of UV laser diodes, their lifetime is very limited.
To obtain deep-UV light, the light output by a solid state laser may be modified by a frequency doubling crystal. Efficient frequency-doubling by a non-linear crystal requires a coherent light source with high output power and Gaussian beam profile. Highest conversion efficiencies have been achieved when the non-linear crystal is placed in a resonator cavity using the enhanced optical field strength at the anti-nodes of a standing optical wave within the resonator (intra-cavity frequency doubling).
In order to achieve deep-UV emission by a single application of frequency doubling, nitride-based laser diodes emitting between 400 nm and 500 nm could be used. The available laser diodes in ridge-waveguide geometry, however, have an inherently low beam quality and are up to now limited in output power. Nitride-based VCSEL have only recently been reported with room-temperature operation and are anticipated to be low-power devices due to insufficient lateral current spreading impeding large area devices. Achieving deep-UV emission using a more widely available gallium-arsenide-based solid state laser effectively requires two applications of frequency doubling. However, the efficiencies of a two-stage frequency doubling significantly reduce output power and increase the size and complexity of the light-producing apparatus.
Recently, the concept of Vertical External-Cavity Semiconductor Lasers (VECSELs) or Semiconductor Disk Lasers (SDL) has been demonstrated as an efficient way for single-step frequency-doubling of near-infrared or red emission into the visible or near-UV spectral range. In SDLs, the excitation area and density are scaled by the diameter and power of an incident pump beam, and the pump power is absorbed over a length scale much larger than in electrically pumped lasers. Therefore, more optically active layers can be incorporated in the lasing process and consequently more output power can be obtained. For a given configuration, if not limited by thermal roll-over, the output power is limited only by the onset of catastrophic optical mirror damage (COMD). In a SDL, a resonator is formed between a semiconductor gain structure, which comprises the light emitting active region and a highly reflective Distributed Bragg Reflector (DBR) mirror, and an external mirror that is partially transparent for laser output. These devices are highly suitable for frequency doubling because of their inherent high beam quality (M2˜1), their high output power (multi-Watt operation) and their compatibility with intra-cavity frequency doubling. Intra-cavity frequency doubling takes advantage of the orders-of-magnitude higher field strength inside the cavity as compared to the field strength available in a tandem-cavities arrangement. Due to the quadratic dependence of the conversion effect on the optical field strength, intra-cavity frequency doubling yields higher optical output by second harmonic generation. A pre-requisite for intra-cavity frequency doubling is having sufficient stability of the fundamental laser operation to withstand insertion of a non-linear optical crystal that inevitably introduces additional losses.
So far, the semiconductor gain structures being used are almost exclusively based on the gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) system. Unlike most other semiconductor material systems the nearly strain-free GaAs/AlGaAs material system enables the growth of high-reflectivity multi-layer stacks out of GaAs/AlGaAs layers and the light emitting layers such as InGaAs quantum wells in a single epitaxial process. However, high-power deep-UV emission is reached only by two stages of frequency doubling from the fundamental infrared wavelength. Direct optical pumping of the quantum wells (QWs) in a SDL structure has also been demonstrated. This approach provides a reduced thermal load on the chip due to carrier relaxation processes and reduced energy deficit provided by the smaller difference of pump and SDL emission energies. However, direct absorption in QWs is inefficient due the small interaction length of the pump light and the QWs, and because of the non-resonant absorption leading to reduced oscillator-strength. If the microcavity exhibits fundamental resonance simultaneously at both the lasing wavelength and a secondary resonance at pump wavelength the absorption efficiency is greatly enhanced. This is achievable in two ways. One way is to use the shift of the resonance frequency of the microcavity towards shorter wavelengths for off-normal incidence angles. Another way is to use cavity thickness for which resonances exists at both the pump wavelength and the laser wavelength at normal incidence.
As compared to the simplicity of conventional laser diodes, SDLs are realized at the expense of an aligned optical setup comprising at least the pump laser diode, the semiconductor gain structure and the out-coupling mirror. The alignment of the setup basically comprises matching the pump laser spot to the width of the laser beam in the SDL cavity, which is determined by the curvature of the external mirror. However, this implies that the SDL apparatus is relatively large and sensitive to mechanical stress. For many applications, for example portable devices, compactness and robustness of the setup are important issues.
While there are motivators to extend what is currently known about arsenide-based systems to the nitride-based materials, there are many hurdles in doing so. Compact, high-power pump sources available to date have emission energies of 3.0 to 2.8 eV (405-445 nm wavelengths) which are lower than the GaN bandedge (3.4 eV). In addition, the energy deficit for pumping at 3.4 eV of a 2.7-2.6 eV emitting SDL amounts already to a 20% efficiency loss. Therefore, optical pumping at a high-power and above the GaN bandedge (similar to GaAs-based SDLs) is not practicable.
Using a pump source with bandedge energy (wavelength) below that of GaN requires that the gain structure be designed for resonant absorption at pump wavelength. As mentioned, this can be done in two ways (tilted incidence of pump light or particular choice of cavity thickness). However, depending on the available pump source wavelength with respect to the lasing wavelength of the SDL, one of either gain structure designs is favored for reasons of either simplicity of pump source-gain structure alignment or positioning of QWs within the gain structure.
GaN is the dominant matrix material in visible, nitride-based light emitters, and cannot be replaced easily by lower bandgap nitrides of sufficient thickness due to underlying growth mechanisms. Another key issue is the thermal management of the semiconductor chip to provide efficient cooling. Because nitrides are mechanically and chemically resistant materials and usually grown on similarly robust substrates like sapphire, the option of using known processes as for GaAs for substrate removal and re-bonding to a thermal heat sink are not available.
Yet another challenging problem for realization of a nitride-based VECSEL is the implementation of a highly reflecting multi-layer stack into the semiconductor structure chip during the epitaxial process. In order to achieve the required high reflectivity of 99.9% the required high number of strained AlGaN/GaN or unstrained InAlGaN/GaN layer pairs (e.g., greater than 60) will degrade the surface quality due to cracking or roughness and will eventually impair laser operation. Moreover, the width of the high reflectivity stop band of such epitaxially grown mirrors is rather small making it difficult to obtain high reflectivities both at pump wavelength and lasing wavelength.
Thus, while it is desirable to employ the techniques utilized in arsenide-based systems with nitride-based devices, hurdles including (a) large differences in materials properties between arsenide- and nitride-based systems, (b) limited availability of high-power pump sources, and (c) large differences in manufacturing of devices, mean that the simple substitution of material systems is not an option.